Allelic variation in the Arabidopsis TNL CHS3/CSA1 immune receptor pair reveals two functional cell-death regulatory modes

Some plant NLR immune receptors are encoded in head-to-head sensor-executor pairs that function together. Alleles of the NLR pair CHS3/CSA1 form three clades. The clade 1 sensor CHS3 contains an integrated domain (ID) with homology to regulatory domains, which is lacking in clades 2 and 3. In this study, we defined two cell-death regulatory modes for CHS3/CSA1 pairs. One is mediated by ID domain on clade 1 CHS3, and the other relies on CHS3/CSA1 pairs from all clades detecting perturbation of an associated pattern-recognition receptor (PRR) co-receptor. Our data support the hypothesis that an ancestral Arabidopsis CHS3/CSA1 pair gained a second recognition specificity and regulatory mechanism through ID acquisition while retaining its original specificity as a guard against PRR co-receptor perturbation. This likely comes with a cost, since both ID and non-ID alleles of the pair persist in diverse Arabidopsis populations through balancing selection.

Automated summary

This study reveals two cell-death regulatory modes for plant NLR immune receptors CHS3/CSA1 pairs, one mediated by the ID domain and the other relying on detecting perturbation of an associated co-receptor. The ancestral Arabidopsis CHS3/CSA1 pair likely gained a second recognition specificity and regulatory mechanism through ID acquisition while balancing selection maintains ID and non-ID alleles.